Magnetic recording medium having a cobalt-based alloy film for high density recording and magnetic storage device using same

ABSTRACT

In one embodiment, a magnetic recording medium comprises an underlying film, a magnetic film and a protective film formed in this order on a substrate. The magnetic film is a cobalt-base alloy film containing chromium and has a plurality of magnetic layers stacked without interposition of any non-magnetic layer. The plural magnetic layers comprise first, second and third magnetic layers. The first magnetic layer is disposed between the underlying film and the second magnetic layer. The second magnetic layer is disposed between the first magnetic layer and the third magnetic layer. The third magnetic layer is disposed between the second magnetic layer and the protective film. The concentration of chromium contained in the first magnetic layer is lower than that of chromium contained in the second magnetic layer. The thickness of the first magnetic layer is smaller than that of the second magnetic layer. The magnetic layers which overlie the first magnetic layer further contain platinum and boron. The concentration of chromium contained in the third magnetic layer is lower than that of chromium contained in the second magnetic layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority from Japanese Patent Application No. JP2004-372388, filed Dec. 24, 2004, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a magnetic recording medium forattaining high-density magnetic recording and a magnetic storage devicecombined with a magnetic storage head.

The demand for a larger capacity magnetic disk drive is becoming moreand more keen. To meet this demand it is required to develop a magnetichead of high sensitivity and a magnetic recording medium having a highS/N ratio. To improve the S/N ratio of a magnetic recording medium it isnecessary to improve the read output for high-density recording.Generally, the magnetic recording medium comprises a first underlyinglayer called a seed layer formed on a substrate, a second underlyinglayer of a body-centered cubic structure formed of an alloy containingchromium as a main component, a magnetic film, and a protective filmcontaining carbon as a main component. The magnetic film is formed of analloy with a hexagonal close-packed structure containing cobalt as amain component. To improve the read output it is effective to let themagnetic film have a crystal orientation such that (110) plane or (100)plane is nearly parallel to the substrate surface and let the c-axis ofthe hexagonal close-packed structure be an axis of easy magnetizationfacing in the in-plane direction. It is known that the crystalorientation of the magnetic film can be controlled by the seed layer. Asa technique for attaining both thermal stability and low noise there isdisclosed in Patent Literature 1 (Japanese Patent Laid-open No. 7-134820(p. 3)) a magnetic recording medium wherein an underlying layer isformed on a substrate and at least two magnetic layers of differentcompositions are formed in contact with each other on the underlyinglayer to constitute a laminated magnetic film, the laminated magneticfilm being provided in multiple layers through a non-magnetic layer suchas a ruthenium layer. As a technique for improving the crystallographicorientation of a magnetic layer there is disclosed in Patent Literature2 (U.S. Pat. No. 6,150,015) a magnetic thin film having an ultra-thinnucleating layer formed of a Co—Cr-base alloy deposited at a lowdeposition rate of not higher than 1 Å/sec. and a recording layer formedof a Co—Cr—Pt-base magnetic alloy deposited at a high deposition rate.As a technique for improving the output characteristic of a magneticrecording medium there is disclosed in Patent Literature 3 (JapanesePatent No. 3576372) a magnetic recording medium comprising anon-magnetic underlying film, a magnetic film and a protective filmformed in this order on a substrate, the non-magnetic underlying filmbeing formed of a Cr or Cr alloy, the magnetic film comprising aplurality of magnetic layers of a Co alloy containing Cr, the magneticlayers having Cr contents which become gradually lower from lower toupper magnetic layers.

BRIEF SUMMARY OF THE INVENTION

As a result of studies made by the present inventors it turned out thatthere were the following problems in the prior art. Improving the readoutput and also reducing the noise of a recording medium are importantfor improving the S/N ratio of the recording medium. To reduce therecording medium noise it is effective to form multiple layers, make thegrain size microfine and decrease Brt. Brt is the product of remanence(Br) and the thickness (t) of magnetic layers. However, making the grainsize microfine and decreasing Brt both to an extreme degree would causedeterioration of the thermal stability and thus a limit is encounteredin the reduction of noise. Therefore, it is also necessary to study thehigher coercivity of the recording medium. By making the coercivityhigh, it is possible to improve a half-value width PW50 of an isolatedread wave output, but there arises a problem in that the overwritecharacteristic may be deteriorated.

According to the technique disclosed in Patent Literature 1, Brt can beset low while maintaining the magnetic film thickness large incomparison with a recording medium formed by a single magnetic layer.However, it is still insufficient to attain a surface recording densityof 95 Mbit or more per square millimeter. Thus, it is necessary tofurther improve the read output and decrease the recording medium noise.

In Patent Literature 2, the crystallographic orientation of a magneticlayer can be enhanced, but the effect of improving the crystallographicorientation by decreasing the film deposition rate is less significantand it is insufficient to attain a surface recording density of 95 Mbitor more per square millimeter of film. Thus, it is necessary to improvethe read output and decrease the recording medium noise. The filmformation using a low deposition rate of 1 Å/sec. or less is notrealistic from the standpoint of productivity and discharge stability.

In Patent Literature 3, although the output characteristic is enhanced,it is insufficient to attain a surface recording density of 95 Mbit ormore per square millimeter. It is necessary to optimize the compositionand thickness of the magnetic layer formed in contact with theunderlying layer and improve the crystallographic orientation of themagnetic layer and make crystal grains microfine, thereby reducing therecording medium noise.

It is a feature of the present invention to provide a longitudinalmagnetic recording medium having a high recording medium S/N ratio,nonproblematic overwrite characteristics, a superior bit error rate andfull stability to thermal fluctuation. It is another feature of thepresent invention to provide a highly reliable magnetic storage devicewhich, when combined with a magnetic head of high sensitivity, canattain a surface recording density of 95 Mbit or more per squaremillimeter. In particular, the present invention aims at obtaining acomposition and thickness of a magnetic film optimum for improving theoutput characteristic while reducing noise in a recording medium.

Typical modes of the invention as disclosed herein will now be outlined.A magnetic recording medium of the invention includes an underlyingfilm, a magnetic film and a protective film, which are formed in thisorder on a substrate. The magnetic film is a cobalt-base alloy filmcontaining chromium and includes a plurality of magnetic layers whichare stacked without interposition of a non-magnetic layer. The pluralityof magnetic layers comprise first, second and third magnetic layers, thefirst magnetic layer being disposed between the underlying film and thesecond magnetic layer, the second magnetic layer being disposed betweenthe first and third magnetic layers, the third magnetic layer beingdisposed between the second magnetic layer and the protective film. Theconcentration of chromium contained in the first magnetic layer is lowerthan that of chromium contained in the second magnetic layer, and thethickness of the first magnetic layer is smaller than that of the secondmagnetic layer. The second and third magnetic layers each furthercontain platinum and boron, and the concentration of chromium containedin the third magnetic layer is set to a level lower than that ofchromium contained in the second magnetic layer. Preferably, thethickness of the first magnetic layer is set at a value of not smallerthan about 0.8 nm and not larger than about 2.0 nm.

According to the present invention it is possible to provide alongitudinal magnetic recording medium having a high recording mediumS/N ratio, nonproblematic overwrite characteristics, a superior biterror rate, and full stability to thermal fluctuation. Further, it ispossible to provide a highly reliable magnetic storage device which,when combined with a magnetic head of high sensitivity, can attain asurface recording density of 95 Mbit or more per square millimeter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural sectional view of a magnetic recording medium ina working example of the present invention.

FIG. 2 shows characteristics of heads used in measurement.

FIG. 3 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 4 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 5 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 6 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 7 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 8 shows characteristics of heads used in measurement for certainsamples.

FIG. 9 shows characteristics of heads used in measurement for certainsamples.

FIG. 10 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 11 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 12 shows electromagnetic conversion characteristics in a workingexample of the present invention.

FIG. 13 shows a magnetic disk drive using a magnetic recording mediumand a magnetic head according to the present invention.

FIG. 14 is a schematic perspective view showing the structure of themagnetic head.

DETAILED DESCRIPTION OF THE INVENTION

Working examples of the present invention will be described in detailwith reference to the drawings. FIG. 13 shows an example of a magneticstorage device to which magnetic recording media and magnetic headsdescribed in the following working examples are applied. The magneticstorage device includes a magnetic recording medium (disk) 110, a diskfixing mechanism 120, a ramp mechanism 140, a voice coil motor (VCM)160, a head stack assembly (HSA) 150, and a magnetic head 100. FIG. 14is a schematic perspective view showing the structure of the magnetichead. The magnetic head 100 is a composite head having an inductive headfor write and a magnetoresistive head for read both formed on asubstrate 601. The recording head includes an upper magnetic pole piece603 and a combined lower magnetic pole and upper shield layer 604 in asandwiching relation to coils 602. Write width and write gap length arehere designated Tww and Gl, respectively. The read head comprises amagnetoresistive sensor 605 and electrode patterns 606 formed at bothends of the sensor 605. The magnetoresistive sensor is sandwiched inbetween the combined lower magnetic pole and upper shield layer 604 anda lower shield layer 607. A read track width and the distance betweenthe two shield layers are here designated Twr and Gs, respectively. Inthis drawing, a gap layer between the magnetic pole pieces and gaplayers between the shield layers and the magnetoresistive sensor areomitted. As shown in FIG. 1, the recording medium 110 according to thepresent invention has a film structure comprising underlying films (11,12, 13), a plurality of magnetic layers (a first magnetic layer 14, asecond magnetic layer 15, a third magnetic layer 16), a protective film17 and a lubricant film 18 which are deposited in this order on asubstrate 10.

As the substrate it is preferable to use a chemically strengthened glasssubstrate or a rigid substrate formed by plating a phosphorus-containingnickel alloy to an aluminum alloy. To impart magnetic anisotropy it ispreferable to apply fine texturing in the substantially circumferentialdirection of a disc on the substrate. As a result of having measured asurface roughness in the radial direction of the disc under anintermittent contact type atomic force microscope and with respect tothe size of 5 μm square, it has turned out that satisfactory flyingreliability could be obtained by using a substrate of 2.68 to 4.2 nm interms of a maximum height Rmax and 0.23 to 0.44 nm in terms of anaverage surface roughness.

An underlying film is formed between the substrate and the firstmagnetic layer, whereby it is possible to control the crystallographicorientation of the magnetic film and make crystal grains microfine.Here, a first underlying layer of any one of Ti—Co alloy, Ti—Co—Ni alloyand Ni—Ta alloy, a second underlying layer of W—Co alloy or Ta and athird underlying layer of a body centered cubic structure formed ofCr—Ti—B alloy or Cr—Ti alloy are provided between the substrate and thefirst magnetic layer.

The plural magnetic layers are stacked in three or more layers withoutinterposition of a non-magnetic layer such as a ruthenium layer. Thatis, an intermediate layer for antiferromagnetic coupling betweenmagnetic layers is not disposed between the magnetic layers, butmagnetic layers are sputtered continuously. The material of the firstmagnetic layer is a cobalt-based alloy not containing platinum, tantalumand boron all together, such as Co—Cr alloy, Co—Cr—Pt alloy, Co—Cr—Balloy, or Co—Cr—Ta alloy. In particular, a Pt-containing alloy ispreferred to stabilize the crystal growth because it is inert even whenformed into a thin film.

The concentration of Cr contained in the second magnetic layer may behigher than that of Cr contained in the first magnetic layer and theconcentration of Cr contained in the second magnetic layer may be lowerthan that of Cr contained in the third magnetic layer. In this case,lattice matching between the underlying film and the first magneticlayer is improved and so is lattice matching between the first andsecond magnetic layers and between the second and third magnetic layers.At the same time, in-plane orientation of magnetic properties isimproved and it is also possible to improve the crystallographicorientation. Thus, the concentrations are more preferred for diminishingthermal fluctuation and attaining a high output resolution and a highrecording medium S/N ratio.

The coercivity of the recording medium is ensured by incorporating Pt inthe second and subsequent magnetic layers. Likewise, by incorporating Bin the second and subsequent magnetic layers, the crystal grain size inthe magnetic layers is made microfine and the recording medium noise isreduced.

The magnetic recording medium is formed on the substrate by sputtering atarget. As a physical vapor deposition method, not only DC sputtering,but also high-frequency sputtering or DC pulse sputtering is alsoeffective. In a case of adopting the DC sputtering method, it ispreferable to apply a bias voltage in the process of forming the secondand subsequent magnetic layers to increase the coercivity. In a magneticstorage device comprising the magnetic recording medium fabricated inthe above manner, a driver for driving the magnetic recording medium, amagnetic head comprising a write section and a read section, a mechanismfor moving the magnetic head relatively with respect to the magneticrecording medium, a mechanical section for unloading the head to a ramp,a signal input unit for the magnetic head, and a read/write signalprocessing unit for read of an output signal from the magnetic head, ifthere is used as the magnetic head a magnetic head having a read sectionconstituted by a magnetoresistive sensor, the magnetoresistive sensorincluding a plurality of electrically conductive magnetic films whichundergo a relative change by the action of an external magnetic fieldand induce a large change in resistance and an electrically conductivenon-magnetic film disposed between the electrically conductive magneticfilms, it becomes possible to attain a surface recording density of 95Mbit or more per square millimeter.

EXAMPLE 1

FIG. 1 shows a sectional structure of a magnetic recording mediumaccording to Example 1 of the present invention. An aluminosilicateglass substrate 10 having a chemically strengthened surface was washedwith alkali and then dried. Thereafter, a 15-nm-thick layer of Ti-40 at.% Co-10 at. % Ni alloy as a first underlying layer 11 and a 3-nm-thicklayer of W-30 at. % Co alloy as a second underlying layer 12 were formedat room temperature. The substrate was heated with a lamp heater so thatthe temperature thereof rose to about 400° C., followed by formation ofan 8-nm-thick layer of Cr-10 at. % Ti-3 at. % B alloy formed as a thirdunderlying layer 13. Further, a first magnetic layer (M1) 14 of Co-16at. % Cr-9 at. % Pt alloy having a thickness of 0.6 to 3 nm, a secondmagnetic layer (M2) 15 of Co-23 at. % Cr-13 at. % Pt-5 at. % B-2 at. %Ta alloy having a thickness of 10 nm and a third magnetic layer (M3) 16were formed in this order, followed by formation of a 3-nm-thick film asa protective film containing carbon as a main component. After theformation of the carbon film, a 1.8-nm-thick lubricant film 18 wasformed by the application of a lubricant containing a perfluoroalkylpolyether as a main component. The following alloys were used for theformation of third magnetic layers:

Co-12 at. % Cr-13 at. % Cr-12 at. % B

Co-18 at. % Cr-13 at. % Cr-8 at % B

Co-12 at. % Cr-12 at. % Cr-10 at. % B-2 at. % Cu.

The above multi-layer film was formed using a single wafer sputteringapparatus. A base vacuum degree of this sputtering apparatus was1.0˜1.2×10⁻⁵ Pa and tact was set at 9 seconds. The first underlyinglayer up to the third magnetic layer were formed in an Ar gas atmosphereof 0.93 Pa. Heating was performed in a mixed gas atmosphere comprisingAr and 1% oxygen added thereto and the protective film of carbon wasformed in a mixed gas atmosphere comprising Ar and 10% nitrogen addedthereto. During sputtering of the third underlying layer and the secondand third magnetic layers, a bias voltage of −200V was applied to thesubstrate. The discharge time of the first underlying layer and thesecond and third magnetic layers was set at 4.5 seconds, that of thesecond underlying layer and the first magnetic layer was set at 2.5seconds, and that of the third underlying layer was set at 4.0 seconds.Brt (Br: magnetic layer remanence, t: magnetic layer thickness) andremanence coercivity Hcr were evaluated using a Fast Remanent MomentMagnetomer (FRMM) with respect to each recording medium fabricated.KV/kT (K: crystal magnetic anisotropy constant, V: volume of magneticcrystal grains, k: Boltzmann constant, T: absolute temperature) wasdetermined by fitting the time dependency of remanence coercivity from7.5 to 240 seconds at room temperature into Sharrock's expression withuse of a vibrating sample magnetometer (VSM). According to the studiesmade by the present inventors, if the value of KV/kT obtained by thismethod is approximately 70 or more, it is possible to suppress outputattenuation caused by thermal fluctuation, so that reliability is notaffected.

Evaluation of electromagnetic conversion characteristic was made in spinstand mode by combination with a composite head having both an inductivemagnetic head for write and a spin valve type magnetic head for read.Characteristics of heads used in this Example and other Examples areshown in FIG. 2. Maximum linear recording density HF (kFC/mm), recordingcurrent Iw (mA), sense current Is (mA), write track width Tww (μm), readtrack width Twr (μm), skew angle Skew (deg.), and the number ofrevolutions ROTNUM (s⁻¹), of each of head samples HEAD No. 1-10 areshown in FIG. 2. Head sample (HEAD No.) 1 was used for evaluation ofthis Example. Recording was performed at a high recording density HF andnormalized noise (kNdhf) was evaluated. A signal to noise ratio Smf/Nwas determined from an output obtained by recording at a mediumrecording density MF=HF/2 and a recording medium noise at the highrecording density HF. After recording at a low recording densityLF=HF/10, a high recording density HF signal was overwritten and anoverwrite characteristic 0/W was determined from an attenuation ratio ofan LF signal. A bit error rate (BER) was determined by counting thenumber of error bytes relative to the total number of read bytes whenread was performed just after a nearly one-round recording for aspecific track using a random pattern. PW50 stands for a half-valuewidth of an isolated read wave.

Evaluation results of SAMPLE No. 101-124 are shown in FIG. 3. In thesame figure there are shown Brt (Tnm), Hcr (kA/m), KV/kT, PW50 (nm), 0/W(dB), kNdhf, Smf/N (dB), and a logarithm of BER, logBER, of recordingmedia obtained by changing composition COMP and thickness (TH1, TH3) ofeach of the first magnetic layer MAGLAY1 and the third magnetic layerMAGLAY3.

For the thickness dependency of the first magnetic layer MAGLAY1, Brtincreased with an increase in thickness TH1 of the first magnetic layerMAGLAY 1. Hcr became maximum at a thickness of the first magnetic layerof about 0.8 nm and greatly decreased at a first magnetic layerthickness of 1.5 nm or more, thus exhibiting a great decrease with anincrease of the thickness. When the thickness of the first magneticlayer was in the range of 1.0 to 1.5 nm, kNdhf decreased to a minimum,Smf/N was improved, and BER became small. Even when the thickness of thefirst magnetic layer was in the range of about 0.8 to 2.0 mm, there wereobtained satisfactory Smf/N and BER close to those obtained at athickness of 1.0 to 1.5 nm. kNdhf, Smf/N and BER exhibited a conspicuousdeterioration at first magnetic layer thicknesses of 0.6 nm and 2.5 nmor more. However, KV/kT was improved with an increase in thickness ofthe first magnetic layer. In the case where the thickness of the firstmagnetic layer was as thin as 0.6 nm, it is probable that thedeterioration in crystallographic orientation of the second andsubsequent magnetic layers grown on the first magnetic layer led todeterioration of the electromagnetic conversion characteristic. It canbe said that the conspicuous deterioration at a first magnetic layerthickness of about 2.0 nm or more is because of a great decrease of Hcr.

A comparison will now be made about the composition of the thirdmagnetic layer MAGLAY3 in the case where the thickness of the firstmagnetic layer is in the range of 1.0 to 1.5 nm. In recording mediahaving a relatively high Cr concentration of the third magnetic layerlike test examples (SAMPLE No.) 112 to 114, kNdhf was smaller than inthe other compositions. In recording media having a relatively low Crconcentration of the third magnetic layer like test examples (SAMPLENo.) 104 to 106 and 120 to 122, PW50 was smaller than and Smf/N wasequal to or higher than in test examples (SAMPLE No.) 112 to 114 havinga high Cr concentration. That is, the following facts became clear. Inthe case of selecting a composition with a high Cr concentration as thecomposition of the third magnetic layer, it is possible to reduce therecording medium noise, but the output characteristic is deteriorated.Conversely, in the case of selecting a composition with a low Crconcentration as the composition of the third magnetic layer, the outputcharacteristic is improved, but the recording medium noise increases.From a comparison between test examples (SAMPLE No.) 104 to 106 and testexamples (SAMPLE No.) 120 to 122, both having an equal Cr concentration,it turned out that test examples (SAMPLE No.) 104 to 106 high in theconcentration of B were lower in kDdhf and larger in Smf/N. It is seenthat a higher concentration of B is also effective in reducing noise. Inthe case of adding Cu as in test examples (SAMPLE No.) 120 to 122, itwas possible to ensure sufficient Hcr despite the concentration of Ptbeing low. Thus, it is seen that the addition of Cu is effective inimproving Hcr. In this Example, the recording media using Co-12 at. %Cr-13 at. % Pt-12 at. % B as the material of the third magnetic layerexhibited the best BER, but no significant difference was recognizedwith respect to the other compositions.

Magnetic recording media not having the first magnetic layer were alsofabricated, all of which could not afford an output signal in evaluationusing FRMM and hence made it impossible to evaluate magnetic properties.This is probably due to the lack of in-plane preferential orientation ofthe second and third magnetic layers in the absence of any firstmagnetic layer.

Magnetic recording media not having the third magnetic layer were alsofabricated, all of which decreased 100 kA/m or more in Hcr and had aserious problem in thermal stability. This is probably because thesecond magnetic layer is high in Cr concentration and low in crystalmagnetic anisotropy and therefore it became impossible to maintainsufficient coercivity in the absence of the third magnetic layer lowerin Cr concentration and higher in crystal magnetic anisotropy than thesecond magnetic layer.

Therefore, the first magnetic layer is essential for preferentialin-plane orientation of the second and third magnetic layers. Theformation of a second magnetic layer having a high Cr concentration iseffective for the attainment of low Brt and low noise, but a high Crmagnetic layer is low in coercivity and cannot ensure thermal stability.Further, in the case of forming a third magnetic layer higher in Crconcentration than the second magnetic layer, it is also impossible toobtain sufficient coercivity and thermal stability cannot be ensured.Therefore, after the formation of a second magnetic layer having a highCr concentration, it is necessary to form a third magnetic layer lowerin Cr concentration than the second magnetic layer in order to increasecoercivity and ensure thermal stability. It is generally known that, inthe case of forming a magnetic film by plural magnetic layers, theoutput characteristic is improved by making the ratio of a ferromagneticmaterial such as cobalt to a nonmagnetic material such as chromium highin upper layers to increase magnetization. Platinum exhibitsferromagnetism by being incorporated in a cobalt-base alloy. Therefore,by making the concentration of cobalt and that of platinum higher inupper layers than the second magnetic layer, it becomes possible toimprove the output characteristic.

The substrate 10 may be a substrate having an outside diameter of 84 mm,inside diameter 25 mm, thickness 1.27 mm, maximum height Rmax 3.5 nm,and average surface roughness Ra 0.35 nm, or a substrate having anoutside diameter of 65 mm, inside diameter 20 mm, thickness 0.635 mm,Rmax 2.68 to 4.0 nm, and Ra 0.23 to 0.44 nm. There is no restriction onthe shape of the substrate 10. Surface roughness in the radial directionof the disc was determined by observing the size of 5 μm square under anintermittent contact type atomic force microscope.

As the material of the first underlying layer there may be used Ti-50at. % Co alloy or Ni-38 at. % Ta alloy. From the standpoint of slidereliability it is preferable that the thickness of the first underlyinglayer be greater than 10 nm. Likewise, from the standpoint ofproductivity it is preferable that the thickness of the first underlyinglayer be not greater than about 30 nm. A microcrystalline or amorphousmetallic thin film having a composition other than the compositionsdescribed above is also employable.

Ta may be used as the material of the second underlying layer. If thesecond underlying layer is formed too thick, its mechanical reliabilitywill be deteriorated and therefore it is preferable that the thicknessof the second underlying layer be 5 nm or smaller.

Cr—Ti alloy not containing B may be used as the material of the thirdunderlying layer. To make crystal grains microfine in a dischargeatmosphere in which oxygen and nitrogen are not added intentionally, itis preferable to add boron to the third underlying layer. A suitableconcentration of boron to be added can be selected so as to afford adesired value of coercivity. If boron is added in an amount exceeding 10at. %, the crystal grains will become microfine to an excessive degree.

The first magnetic layer may be an alloy not containing platinum,tantalum and boron all together, such as Co—Cr alloy, Co—Cr—B alloy,Co—Cr—Pt alloy, or Co—Cr—Ta alloy. It is preferable that theconcentration of Cr added be in the range of 10 at. % to 20 at. %. Inparticular, an alloy not containing Pt is preferable in point ofstabilizing the crystal growth because the surface thereof is relativelyinert even when formed into a thin film.

EXAMPLE 2

Magnetic recording media were formed in the same way as in Example 1except that the compositions and thicknesses of the first, second andthird magnetic layers and the substrate temperature were changed. ACo-22 at. % Cr-14 at. % Pt-4 at. % B-2 at. % Ta alloy layer having athickness of 10 nm was formed as the second magnetic layer and a Co-11at. % Cr-13 at. % Cr-15 at. % B alloy layer having a thickness of 8 nmwas formed as the third magnetic layer. The following alloys were usedas the materials of first magnetic layers each formed of a Co-basealloy:

Co-16 at. % Cr-9 at. % Pt

Co-34 at. % Cr.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 2 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 201 to 206 are shown in FIG. 4. InSAMPLE Nos. 204 to 206 wherein Co-34 at. % Cr higher in Cr concentrationthan the second magnetic layer was used as the first magnetic layerMAGLAY1, Smf/N was deteriorated by about 1 dB and BER was alsodeteriorated by an order of about 0.5 to 1.0 in comparison with SAMPLENos. 201 to 203 in which Co-16 at. % Cr-9 at. % Pt lower in Crconcentration than the second magnetic layer was used as the firstmagnetic layer MAGLAY1. That is, it can be said that the electromagneticconversion characteristic is good if the Cr concentration of the firstmagnetic layer is lower than that of the second magnetic layer.

As comparative examples, magnetic recording media using Cr—Mo alloylayers instead of Co-base alloys of the first magnetic layers were alsoformed. As Cr—Mo alloys there were provided two compositions of Cr-30at. % Mo and Cr-40 at. % Mo. Thicknesses of the Cr—Mo alloy layers wereset at 1.0 mm, 1.5 mm, and 2.0 nm. In the case of using Cr—Mo alloys, ascompared with SAMPLE Nos. 201-206, Brt values of the media lowered by 3Tnm or more independently of their compositions and thicknesses. This isprobably because the magnetic layers overlying the Cr—Mo alloy layersdid not exhibit in-plane preferential orientation. The first magneticlayer should be formed of a Co-base alloy in order to attain in-planepreferential orientation of the second and subsequent magnetic layers.

EXAMPLE 3

Magnetic recording media were formed in the same way as in Example 1except that the compositions and thicknesses of the first underlyinglayer and the first magnetic layers were changed. As the material of thethird magnetic layer there was selected Co-12 at. % Cr-13 at. % Pt-12at. % B. A Ti-50 at. % Co alloy layer having a thickness of 15 nm wasformed as the first underlying layer. The following alloys were used asthe materials of first magnetic layers:

Co-10 at. % Cr

Co-14 at. % Cr

Co-27 at. % Cr

Co-14 at. % Cr-4 at. % Pt

Co-14 at. % Cr-8 at. % Pt

Co-14 at. % Cr-12 at. % Pt

Co-19 at. % Cr-8 at. % Pt.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 3 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 301 to 321 are shown in FIG. 5. InSAMPLE Nos. 307 to 309 wherein Co-27 at. % Cr higher in Cr concentrationthan the second magnetic layer was used as the first magnetic layer,Smf/N was deteriorated by about 1 to 2 dB and BER was deteriorated by anorder of about 1 in comparison with the other test examples wherein theCr concentration of the first magnetic layer is lower than that of thesecond magnetic layer. As is the case with Example 2, it turned out thatthe electromagnetic conversion characteristic was deteriorated at ahigher Cr concentration of the first magnetic layer than that of thesecond magnetic layer.

When a comparison was made with respect to test examples having a Crconcentration of the first magnetic layer of 14 at. %, BER values inSAMPLE Nos. 310 to 315 containing 4 at. % or 8 at. % of Pt were smallerby an order of about 0.2 than in SAMPLE Nos. 304 to 306 not containingPt. From these results it turned out preferable that the first magneticlayer formed of a Co-base alloy containing Cr further contain Pt.However, in SAMPLE Nos. 316 to 318 containing 12 at. % of Pt, thereoccurred a slight deterioration in BER, etc. Thus, it turned out that anupper limit was encountered in the concentration of Pt added to thefirst magnetic layer for improving the electromagnetic conversioncharacteristic.

EXAMPLE 4

Magnetic recording media were formed in the same way as in Example 1except that different alloy layers were formed as first magnetic layers.Co-12 at. % Cr-13 at. % Pt-12 at. % B was selected as the material ofthe third magnetic layer. The following alloys were used as thematerials of first magnetic layers:

Co-14 at. % Cr

Co-16 at. % Cr

Co-18 at. % Cr

Co-20 at. % Cr

Co-14 at. % Cr-4 at. % Pt

Co-16 at. % Cr-4 at. % Pt

Co-16 at. % Cr-9 at. % Pt.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 4 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 401 to 426 are shown in FIG. 6. InSAMPLE Nos. 401 to 416 using Co—Cr alloys as the materials of firstmagnetic layers, there was no significant difference in kNdhf, Smf/N,and BER. In these test examples, Cr concentrations of first magneticlayers are 14 to 20 at. %, all of which are lower than the Crconcentration of the second magnetic layer. Thus, it turned out that toattain a satisfactory electromagnetic conversion characteristic it wasimportant to set the Cr concentration in each first magnetic layer lowerthan that of the second magnetic layer.

In SAMPLE Nos. 417 to 426 using first magnetic layers each having a Crconcentration of 14 at. % or 16 at. % and containing 4 at. % or 9 at. %of Pt, BER values were smaller by an order of about 0.5 than in SAMPLENos. 401 to 416 not containing Pt in first magnetic layers. As inExample 3, it turned out that the electroconversion characteristic wasfurther improved by the addition of Pt to the first magnetic layers. Italso turned out that an upper limit of the concentration of Pt added forthe improvement of electromagnetic conversion characteristic was higherthan 9 at. %.

EXAMPLE 5

Magnetic recording media were formed in the same way as in Example 1except that the compositions and thicknesses of the first magneticlayers were changed. Co-12 at. % Cr-13 at. % Pt-12 at. % B was selectedas the material of the third magnetic layer. The following alloys wereused as the materials of first magnetic layers. Although nonmagneticmaterials are included among the following alloys, they are heredesignated first magnetic layers for convenience:

Co-16 at. % Cr-9 at. % Pt

Co-40 at. % Ru

Co-50 at. % Ru

Co-40 at. % Ru-3 at. % B

Co-24 at. % Ru-14 at. % B

Co-26 at. % Ru-10 at. % B

Co-20 at. % Cr-30 at. % Ru

Co-20 at. % Cr-40 at. % Ru.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 5 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 501 to 530 are shown in FIG. 7. InSAMPLE Nos. 503 to 530 using first magnetic layers containing Ru,electromagnetic conversion characteristic or magnetic properties weregreatly deteriorated in comparison with SAMPLE Nos. 501 and 502 using afirst magnetic layer of Co-16 at. % Cr-9 at. % Pt. In SAMPLE Nos. 503 to510 using Co—Ru alloys as first magnetic layers and SAMPLE Nos. 511 to514 wherein a small amount of B was added to Co—Ru, relatively goodmagnetic properties were obtained, but Smf/N was deteriorated by 1 dB ormore and BER was also deteriorated by an order of 0.5 or more. In SAMPLENos. 515 to 522 using Co—Ru—B alloys containing 10 at. % or more of B asfirst magnetic layers and also in SAMPLE Nos. 523 to 530 using Co—Cr—Rualloys as first magnetic layers, Brt and Hcr were greatly deteriorated.That the magnetic properties were greatly deteriorated is due to amarked deterioration in crystallographic orientation of the second andsubsequent magnetic layers.

From the above results it can be said that the first magnetic layers notcontaining Ru afford better magnetic properties and electromagneticconversion characteristic.

EXAMPLE 6

Magnetic recording media were formed in the same way as in Example 1except that the composition and thickness of the second magnetic layerand the substrate temperature were changed. The first magnetic layerswere formed at a thickness of 1.2 nm. Co-12 at. % Cr-13 at. % Pt-12 at.% B alloy was selected as the material of third magnetic layers. Thefollowing alloys were used as the materials of second magnetic layers:

Co-22 at. % Cr-14 at. % Pt-4 at. % B-2 at. % Ta

Co-22 at. % Cr-14 at. % Pt-6 at. % B-2 at. % Ta

Co-24 at. % Cr-14 at. % Pt-6 at. % B

Co-22 at. % Cr-14 at. % Pt-8 at. % B

Co-18 at. % Cr-12 at. % Pt-8 at. % B

Co-20 at. % Cr-12 at. % Pt-8 at. % B.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. The results obtained inSAMPLE Nos. 601 to 606 using Head No. 6 (FIG. 2) are shown in FIG. 8,the results obtained in SAMPLE Nos. 601 to 603 and 613 to 618 using HeadNo. 7 (FIG. 2) are shown in FIG. 9, and the results obtained in SAMPLENos. 601 to 603 and 619 to 624 using Head No. 8 (FIG. 2) are shown inFIG. 10.

SAMPLE Nos. 601 to 606 using Co—Cr—Pt—B—Ta alloys as second magneticlayers were equal in electromagnetic conversion characteristicindependently of the concentration of B. In SAMPLE Nos. 604 to 606 usinga B concentration of 6 at. %, KV/kT was smaller than in SAMPLE Nos. 601to 603 using a B concentration of 4 at. %, but the level thereof posesno problem in point of reliability. The reason why the value of KV/kTwas small is probably that the crystal grain size of magnetic layersbecame microfine with an increase of the concentration of B.

In SAMPLE Nos. 613 to 624 using Co—Cr—Pt—B alloys as second magneticlayers and SAMPLE Nos. 613 to 618 having a Cr concentration in secondlayers of 22 at. % or higher, there were exhibited equal levels ofkNdhf, Smf/N and BER to those in SAMPLE Nos. 601 to 603 using Co-22 at.% Cr-14 at. % Pt-4 at. % B-2 at. % Ta alloy as the material of secondmagnetic layers. SAMPLE Nos. 613-618 were smaller in KV/kT than SAMPLENos. 601-603, but the level thereof poses no problem in point ofreliability. The reason why KV/kT was small is probably that the crystalgrain size of magnetic layers became microfine with an increase of theconcentration of B. In SAMPLE Nos. 622 to 624 having a Cr concentrationin second layers of 20 at. %, kNdhf values were equivalent to those inthe above SAMPLE Nos. 601 to 618, but SmF/N and BER were somewhatdeteriorated. In SAMPLE Nos. 619-621 having a Cr concentration in secondmagnetic layers of 18 at. %, kNdhf, Smf/N and BER were deteriorated incomparison with the other test examples. Thus, it turned out that toreduce noise of each magnetic recording medium it was preferable to setthe Cr concentration of the second magnetic layer at about 20 at. % orhigher and that for further improvement of Smf/N and BER it waspreferable to set the Cr concentration of the second magnetic layer at22 at. % or higher.

EXAMPLE 7

A first underlying layer of Ti-50 at. % Co alloy having a thickness of15 nm, a second underlying layer of W-30 at. % Co alloy having athickness of 3 nm, a third underlying layer of Cr-10 at. % Ti-3 at. % Balloy having a thickness of 8 nm, a first magnetic layer of Co-16 at. %Cr-9 at. % Pt alloy having a thickness of 1 nm, a second magnetic layerof Co-23 at. % Cr-13 at. % Pt-5 at. % B-2 at. % Ta alloy having athickness of 11 nm, and a third magnetic layer of Co-11 at. % Cr-13 a. %Pt-15 at. % B alloy were formed in this order on a substrate in the sameway as in Example 1, followed by further formation of a fourth magneticlayer. After the formation of the fourth magnetic layer, a protectivelayer and a lubricant layer were formed in the same manner as inExample 1. The following alloys were used as the materials of fourthmagnetic layers:

Co-12 at. % Cr-13 at. % Pt-12 at. % B

Co-12 at. % Cr-12 at. % Pt-1 Oat. % B-2 at. % Cu.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 9 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 701 to 709 are shown in FIG. 11. InSAMPLE Nos. 702 to 704 and 706 to 708 each having a total of fourmagnetic layers, Smf/N and BER were somewhat improved in comparison withrecording media each having a total of three magnetic layers like SAMPLENo. 701 wherein a fourth magnetic layer was not formed and like SAMPLENos. 705 and 709 wherein a fourth magnetic layer was formed withoutforming a third magnetic layer. It is probable that the outputcharacteristic, as well as Smf/N and BER, was improved by forming afourth magnetic layer high in the concentration of Co and that of Pt andof large magnetization. Thus, it turned out that even in a magneticrecording medium having four or more magnetic layers, the outputcharacteristic and electromagnetic conversion characteristic could beimproved by setting high the sum of the concentration of cobalt and ofplatinum in the layers above the second magnetic layers.

EXAMPLE 8

Magnetic recording media were formed in the same way as in Example 7except that different alloy layers were formed as third and fourthmagnetic layers. The following alloys were used as the materials ofthird magnetic layers:

Co-12 at. % Cr-13 at. % Pt-12 at. % B

Co-18 at. % Cr-13 at. % Pt-8 at. % B.

The following alloys were used as the materials of fourth magneticlayers:

Co-6 at. % Cr-13 at. % Pt-16 at. % B

Co-8 at. % Cr-13 at. % Pt-16 at. % B

Co-8 at. % Cr-13 at. % Pt-14 at. % B

Co-8 at. % Cr-13 at. % Pt-12 at. % B

Co-10 at. % Cr-13 at. % Pt-14 at. % B

Co-10 at. % Cr-13 at. % Pt-12 at. % B

Co-10 at. % Cr-13 at. % Pt-10 at. % B

Co-12 at. % Cr-13 at. % Pt-10 at. % B

Co-10 at. % Cr-6 at. % Pt-4 at. % B.

Magnetic properties and electromagnetic conversion characteristic wereevaluated in the same manner as in Example 1. Head No. 10 (FIG. 2) wasused for the evaluation of electromagnetic conversion characteristic.The results obtained in SAMPLE Nos. 801 to 865 are shown in FIG. 12.Among the magnetic recording media formed in this Example, in all ofthose each having four magnetic layers, approximately equal values ofSmf/N and BER were obtained. All the magnetic recording media eachhaving four magnetic layers in this Example are characterized by afourth magnetic layer having a larger sum of the concentration of Co andof Pt than in a third magnetic layer. It is seen that these magneticrecording media each having four magnetic layers are smaller in PW50than and improved in output characteristic over the magnetic recordingmedia each having three magnetic layers such as SAMPLE Nos. 801 and 838.From a comparison of fourth magnetic layer compositions it is seen thatin the compositions having a Cr—B concentration sum of 22 at. % orlower, i.e., in the compositions having a Co—Pt concentration sum of 78at. % or higher, kNdhf increased a little in comparison with thecompositions having a Co—Pt concentration sum of 76 at. % or lower. Fromthe standpoint of reducing noise it can be said that there is an upperlimit as to the sum of the concentration of Co and that of Pt in eachfourth magnetic layer.

It is to be understood that the above description is intended to beillustrative and not restrictive. Many embodiments will be apparent tothose of skill in the art upon reviewing the above description. Thescope of the invention should, therefore, be determined not withreference to the above description, but instead should be determinedwith reference to the appended claims along with their full scope ofequivalents.

1. A magnetic recording medium comprising an underlying film, a magneticfilm and a protective film, which are formed in order on a substrate;wherein: said magnetic film is a cobalt-base alloy film containingchromium and having a plurality of magnetic layers stacked withoutinterposition of any non-magnetic layer; said plurality of magneticlayers comprising first, second and third magnetic layers; said firstmagnetic layer is disposed between said underlying film and said secondmagnetic layer; said second magnetic layer being disposed between saidfirst magnetic layer and said third magnetic layer; said third magneticlayer being disposed between said second magnetic layer and saidprotective film; the concentration of chromium contained in said firstmagnetic layer is lower than that of chromium contained in said secondmagnetic layer; said first magnetic layer has a thickness smaller than athickness of said second magnetic layer; the magnetic layers whichoverlie said first magnetic layer further containing platinum and boron;and the concentration of chromium contained in said third magnetic layeris lower than the concentration of chromium contained in said secondmagnetic layer; wherein the sum of the concentration of cobalt and theconcentration of platinum contained in said third magnetic layer ishigher than the sum of the concentration of cobalt and the concentrationof platinum contained in said second magnetic layer.
 2. A magneticrecording medium according to claim 1, wherein the thickness of saidfirst magnetic layer is not smaller than about 0.8 nm and not largerthan about 2.0 nm.
 3. A magnetic recording medium according to claim 1,wherein a concentration of chromium contained in said first magneticlayer is not lower than about 10 at. % and not higher than about 20 at.%.
 4. A magnetic recording medium according to claim 1, wherein saidfirst magnetic layer further contains platinum at a platinumconcentration of not higher than about 12 at. %.
 5. A magnetic recordingmedium according to claim 1, wherein said first magnetic layer is acobalt-chromium alloy layer or a cobalt-chromium-platinum layer.
 6. Amagnetic recording medium according to claim 1, wherein theconcentration of chromium contained in said second magnetic layer is notlower than about 20 at. %.
 7. A magnetic recording medium according toclaim 1, wherein said plural magnetic layers further comprise a fourthmagnetic layer formed over and in contact with said third magneticlayer; and wherein the sum of the concentration of cobalt and theconcentration of platinum contained in said third magnetic layer islower than the sum of the concentration of cobalt and the concentrationof platinum contained in said fourth magnetic layer.
 8. A magneticrecording medium comprising at least a first magnetic layer, a secondmagnetic layer and a third magnetic layer, which are formed in order ona substrate through an underlying film; wherein: said first magneticlayer is a cobalt-chromium alloy layer or a cobalt-chromium-platinumalloy layer; said second and third magnetic layers are each acobalt-base alloy layer containing chromium, platinum and boron; saidfirst magnetic layer has a thickness smaller than a thickness of saidsecond magnetic layer; the concentration of chromium contained in saidfirst magnetic layer is lower than the concentration of chromiumcontained in said second magnetic layer; and the concentration ofchromium contained in said third magnetic layer is lower than theconcentration of chromium contained in said second magnetic layer;wherein the sum of the concentration of cobalt and the concentration ofplatinum contained in said second magnetic layer is lower than the sumof the concentration of cobalt and the concentration of platinumcontained in said third magnetic layer.
 9. A magnetic recording mediumaccording to claim 8, wherein the thickness of said first magnetic layeris not smaller than about 0.8 nm and not larger than about 2.0 nm, theconcentration of chromium contained in said first magnetic layer is notlower than about 10 and not higher than about 20at. %, and theconcentration of platinum contained in said first magnetic layer is nothigher than about 12 at. %; and wherein the concentration of chromiumcontained in said second magnetic layer is not lower than about 20 at.%.
 10. A magnetic recording medium according to claim 8, furthercomprising a fourth layer formed over and in contact with said thirdmagnetic layer; wherein said fourth magnetic layer is a cobalt-basealloy layer containing chromium, platinum and boron, and wherein the sumof the concentration of cobalt and the concentration of platinumcontained in said fourth magnetic layer is higher than the sum of theconcentration of cobalt and the concentration of platinum contained insaid third magnetic layer.
 11. A magnetic recording medium comprising,on a substrate, an underlying film, a magnetic film and a protectivefilm; wherein: said magnetic film has a plurality of magnetic layersstacked without interposition of any non-magnetic layer; said pluralityof magnetic layers comprises first, second, third and fourth magneticlayers; said first magnetic layer is disposed between said underlyingfilm and said second magnetic layer; said second magnetic layer isdisposed between said first magnetic layer and said third magneticlayer; said third magnetic layer is disposed between said secondmagnetic layer and said fourth magnetic layer; said fourth magneticlayer is disposed between said third magnetic layer and said protectivefilm; said first magnetic layer is a cobalt-base alloy layer containingchromium; said second, third and fourth magnetic layers are each acobalt-base alloy layer containing chromium, platinum and boron; thethickness of said first magnetic layer is smaller than the thickness ofsaid second magnetic layer; the concentration of chromium contained insaid first magnetic layer is lower than the concentration of chromiumcontained in said second magnetic layer; the concentration of chromiumcontained in said third magnetic layer is lower than the concentrationof chromium contained in said second magnetic layer; and the sum of theconcentration of cobalt and the concentration of platinum contained insaid fourth magnetic layer is higher than the sum of the concentrationof cobalt and the concentration of platinum contained in said thirdmagnetic layer.
 12. A magnetic recording medium according to claim 11,wherein the thickness of said first magnetic layer is not smaller thanabout 0.8 nm and not larger than about 2.0 nm, said first magnetic layerfurther contains platinum, the concentration of chromium contained insaid first magnetic layer is not lower than about 10 at. % and nothigher than about 20 at. %, and the concentration of platinum containedin said first magnetic layer is not higher than 1 about 2 at. %; andwherein the concentration of chromium contained in said second magneticlayer is not lower than about 20 at. %.
 13. A magnetic recording mediumaccording to claim 11, wherein the thickness of said first magneticlayer is not smaller than about 0.8nm and not larger than about 2.0 nm.14. A magnetic recording medium according to claim 11, wherein aconcentration of chromium contained in said first magnetic layer is notlower than about 10 at. % and not higher than about 20 at. %.
 15. Amagnetic recording medium according to claim 11, wherein said firstmagnetic layer further contains platinum at a platinum concentration ofnot higher than about 12at. %.
 16. A magnetic recording medium accordingto claim 11, wherein said first magnetic layer is a cobalt-chromiumalloy layer or a cobalt-chromium-platinum layer.
 17. A magneticrecording medium according to claim 11, wherein the concentration ofchromium contained in said second magnetic layer is not lower than about20 at. %.